What's wrong with this metric conversion for allowable bending stress? 3

[BenJohnson]

I am a newbie here. Long time registered civil engineer in California. Retired now in Indonesia. I am building a house here - a place with no building codes or even the little information available is in a foreign language.

All the wood here is different that good 'ole Douglas fir. Nothing is graded at the lumber yard. I am working with wood like Teak and Mahogany, not for furniture, but for for structural members (joists, truss, etc). The problem is I go to the few web sites that give some properties for wood, and I typically get this:
metric imperial (my conversion)
Crushing strength *: 56 MPa 8,100 PSI
Static bending strength *: 98 MPa 12,900 PSI
Modulus of elasticity *: 13,740 MPa 13,015,700 PSI

I had to do the conversion (shown above) from Mega-pascal to PSI units using the regular metric to imperial conversion calculators. [link

http://www.onlineconversion.com/

]OnlineConversion.com[/url] is my favorite.

Now anybody who has worked with bending stress (Fb) before knows these values are about 10 times too high. Here is the typical values for Northern Red Oak:
Max Fiber Stress 1,600 PSI
Modulus of Elasticity 1.3 Million PSI
Max Shear 205 PSI

These numbers should look familiar to anyone who has gone into the joists and rafter tables to find allowable span.

So my question is, "What am I doing wrong?" All of the woods available in this area show these extremely high strengths - even coconut wood. I don't believe they are 10x as strong as Red Oak. My only guess is the published values for North American wood use a 10 times factor of safety above the laboratory test results. That is hard to believe too. Any ideas?

 

[structSU10]

With wood from the standard NDS they derive the properties from the lowest 5% of samples, then i believe they add additional safety factors. 10 times seems rather large though.

i found this

http://www.forestry.org.au/pdf/pdf-members/afj/AFJ 2007 v70/05Shukla.pdf

near the end there are properties which seem a little more reasonable: 2130 psi extreme fiber stress, etc.

 

[hokie66]

1 MPa is about 145 psi. Your conversion for "crushing strength" is about right, bending strength would be about 14000 psi, and E about 2x10^6 psi. These are reasonable numbers (the strengths are breaking strength), and yes, factors of safety for timber are much higher than for a more predictable material like steel. 6 is a common number.

http://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/3_Wood_Strength/3_Wood_Strength.htm

 

[BenJohnson]

I guess that is the answer... ungraded wood can vary tremendously depending on age and growing conditions, so a large factor of safety is appropriate. Interesting to read the comments from people who have a little more knowledge of the subject. Since my engineering career was totally in the US, the unit of Mega-pascal is something that I seldom encountered - so I have no intrinsic feel when something is about right or dramatically wrong.

Fortunately my project does not have heavy loads, so I can be conservative in my calculations and still be certain modest sized lumber will do the job.

Thanks for your help, guys, and I hope I can be of assistance with other threads in the forum in the future.

 

[woodman88]

The Wood Handbook (a free download at

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p)

gives some values for Teak and Mahogany on pages 5-23 and 5-24. Attached is just chapter 5 of the Handbook.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.